Transmission strip line crossover



June 25, 1963 J. l.. BUTLER 3,095,549

TRANSMISSION STRIP LINE CROSSOVER Filed June 28, 1960 52 4a sa 22 sD\"2\8 Il \I\/ L Fig. 2

Jesse L. Buffler IN VEN TOR www 1 ATTORNEY United States Patent Oliiice3,095,549 TRANSMlssIoN STRIP LINECROSSOVER Jesse L`. ButlerNshua;NJ-l.,assignerV to Sanders Assof ciales, Inc.; Nashua, N.'H:,afcorpdration ofiDelaware This invention relates to the art of highfrequency transmission'lines. f Moreparticularly, it relates to acrossover in which apair of signal paths carried onfstriptr'ansmission"lines intersecteach other without appreciable crosstalk between them. tt

Most electric circuits having 1a fair 1degree of complexity equiresignal paths-to cross over other signal paths without a transfer of`energy from one pathto the other. At low frequencies, this problemcaneasily be solved by use of 1a three dimensional construction in whichone line arches over another vat each crossover. At higher frequencies,`where transmission lines are used and, more specifically, in the case ofstrip lines in which printed or etched circuits are used, Ia threedimensional crossover construction'in which one transmission-line archesover the other is relatively expensive to Jr'abricate. Furthermore, ithas a space requirement which work-s against the present-day trend tominiaturization. y i

-Accor dingly, aprincipal object of my invention A,is to provide animproved ,crossover adaptedfor the tnansposition of two transmissionline signal paths.

Another object fof my invention is to provide fa crossover of the abovecharacter adapted for use in stripl transmi- Ssisn lines, f

A further object is to provide a crossover of the above type lconforminggenerally to -a two 4dimensional layout and therefore adapted -forconstruction byprintedncircut andlike techniques. i l

'Alstill further` objectloflmy invention is to provide a crossoverofthegahove characterfhaving negligible crosstalk between intersectingsign-al paths f V Yetfan'sther object ofthe invention is to provide acrossover of above character adapted rlor relatively inexpensivfaiicnon,"l

Other objects o'f the invention will in part be obvious and will inpatappearV hereinafter. f

The invention accorldinglycomprises the features of construction,combination of elements, and arrangement ofparts"`which'willibeexempliedin the construction hereinafter set forth,and the scope of the invention will be indicated inthe l i In general, atnansir'is'sion line crossover incorporating the yfeatures ofmyinvention comprises a'first `strip thansmission line having a pair ofir'st center conductors disposed between a pair of ground plane'conductors. A secondtraiisrnissio line Crossing 4theiirst line includesa second ce'rte'r conductor also'disposed between the', ground planeconductors. The rst line has first and second sections withthe'center'conductors thereof in registration at theend points of thesesections and diverging from each otherm to loops within the sections.Ineach of the loops, one ofthe center conductors isian oddnurnber ofhalf wavelengths longer than the other. A third sectionofitheiirsttransmission line, in` which the centerconduc- Fors are inregistration, is lconnected between the first and seot-icn'is,`the'center conductor ofithesecond transmission line passes through thefirst line between the center conductors inthe thirdsection.

kFor :a fuller understanding 4ofthe nature and objects of the invention,reference should be `had tothe following detailed description `taken inconnection with the accompanying drawings, in which: f f t `FIGURE 1 isa plan view showing the center conductors of ra pair of strip.transmission lines in a crossover made Iaccording to my invention,

.'FIGURE 2 is a simplified longitudinal vertical section of thecrossoverof FIGURE v1-tal en .along line -2-2 therein, and

FIGURE 3 is -a transverse section taken along line 3-3 of FIGURE/1QV.FIGURE f'1 illustrates a crossover incorporating the principles vor myinvention and includes a pair of center conductors generally indicatedat 22 and `24, belonging to arst trans-mission line, ,and a centerconductor generally indicated at 2.6,v belonging to la second lineintersecting the fstline. The 'conductors 22, 24 and 26 `are disposedbetween a pair of ground plane conductors (not shown in FIGURE 1)."Proceeding -from the left, theconductors 22 and '24 have sections 28 and30 in` registration with each other 4and operatingv as an ordinary stripline. Thus, they may be connected'together by suitable means (not shown)to maintain them at the samepotential. f

Next, the conductor 22 departs from the conductor 24 to form a loop32,while the conductor 24 proceeds in a straight section 34. Theconductors are then in registration in 'intermediate sections 36and 38,following which, one of them, in this case the "conductor 24, describesa loop 40. `The conductor 22 proceeds past the' loop 40 in a section42,and the two conductors iinally returnto registration with eachother insections 44 and 46. The sections 36S-and 3S are insulated from eachother, `although thelother sections are not.

` At each loop, one 'of the two conductors 22 land 24 has a length anodd number of half. wavelengths 4greater than thei'other conductor.Thus, the loop 32 may be a single halffwavelength longer than thesection 34. Accordingly, energy enteringthe crossover by way of thesections 7.8` and 30 in `a normal strip line mode, is split into twoportions travelling along the loop .3 2 and section 34,

respectively, and arriving at the sections 36` and 38 invphasefopposition; Thatis, the voltage land current in the section 36are'-l^80 degrees out of phase with the voltage and current in oppositeportions ofthe section 38. This region `of the transmission'line'therefore operates in a parallel platemode. Finally, Vafter,traversing the loop 40 and'section 42,` another lialfwavelength relativephase change takes place, so that,in the, sections 44 and 46, thevoltages fareiagainin phase `anclequ-al. Therefore, these sections;which'may be connected together, serve as center conductors in 5a'lineloperating in la normal strip line mode. -In the` parallel-plateportions 36,-38 of the transmission line, `an electric iieldV extendsdirectly between the conductors '-22 and V24; and it" follows that'thereis a neutral equipotential plane midway between the two conductors and`,perpendicular to' a line extending between them.

A Thus, this plane is parallel tto-theY strips comprising the conductors22uand'24; theconductor 26,: which is disposed the plane, does notdisturb theiield configuration resulting `from energy `guided by theconductors 22 and 24. In'other words,n'one-oi the energy propagatedalong the Patented June 25, 1963 3 line comprising the latter twoconductors is transferred to the conductor 26.

It should be noted that the same result would not follow if theconductor 26 passed between the conductors 22 and 24 at the sections28-30 or #i4-46, where the line comprising these sections operates in anormal strip line mode with the voltages equal and in phase. The reasonfor this is that there is no field between the conductors 22 and Z4 inthese sections. Rather, the electric fields extend `from theseconductors to the ground plane conductors between which they aresandwiched. The equipotential surfaces are generally elliptic in crosssection with the center conductors in the centers of the ellipses. Acenter conductor passing between them would have to pass through pointsnormally having different potentials, and energy transfer wouldtherefore take place. In fact, for this reason, there is negligibletransfer of energy from the conductor 26 to the conductors 22 and 24.The latter conductors more or less coincide with the flat portions ofthe elliptic equipotential surfaces around the conductor 26 operating ina strip line mode.

Furthermore, energy coupled from the transmission line including thecenter conductor 26 to the line including the conductors 22 and 24 willinduce equal voltages and currents in the latter conductors because ofthe symmetry of their position with respect to the conductors 26. Afterpassing through the loops 32 and 40 and straight sections 34 and 42,this energy `will arrive at the sections 28-30 and 44-46 in phaseopposition between the two conductors (assuming, -for example, that thefrequencies propagated along the two transmission lines are the same).Since the conductors 22 and 24 are shorted together in the sections28-30 and 44-46, energy coupled from the conductor 26 cannot bepropagated along these sections.

It will be noted that the sizes of the conductors 22 and 24 varythroughout the crossover. Thus, proceeding `from the sections 28-30 inwhich a characteristic impedance Z may be assumed, the lines diminish inwidth in the loop 32 and section 34. The line capacitance is thusdecreased and the inductance increased, resulting in an increase incharacteristic impedance. The widths should be such that each conductorprovides a characteristic impedance of 2Z0 in this region. Thus, theparallel combination of the loop 32 and section 34 present an impedanceof Z0 to the section 28-30, and the energy in the latter section dividesequally between the loop and the section 34. The width of the conductors22 and 24 in the section 36-38 should again be such as to provide animpedance match with the preceding section. 'Ilhe same relationshipshold true with the line widths in the loop 40 and section 42 and alsothe section 44--46.

As seen in FIGURES 2 and 3, the crossover is disposed between a pair ofground plane conductors 48 and S0. The ground plane conductors, as wellas the center conductors 22 and 24, are preferably bonded to insulators52 and 54, with the center conductors shaped by conventional printed oretched circuit techniques. Another pair of insulators 56 and `58 aredisposed between the conductors 22 and 24, and the conductor 26 isbonded to one of these latter insulators.

The relative dimensions in the drawings have been exaggerated forpurposes of clarity. Actually, the various conductors are preferably offoil a few mils thick. Also, the space between the conductors 22 and 24is materially less than it would appear from the drawings. For example,it may be on the order of mils.

Thus, I have described a transmission line crossover adapted for usewith transmission lines fabricated by printed circuit techniques. Thecrossover, whichV makes use of strip line techniques, includes a splitcenter conductor configuration for one of the intersecting transmissionlines and a single center conductor for the other. By means of halfwavelength differential sections, the line including a split centerconductor is converted from a 4 strip line mode to a parallel plate modeand then back again. In the parallel plate region, the center conductorof the other line passes between the split conductors in a neutralequipotential plane, thereby providing essentially complete isolationbetween the two lines.

From the above, it will be apparent that a crossover incorporating thefeatures of my invention may be fabricated more easily and compactlythan an arching crossover, particularly in circuits using striptransmission lines.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained and,since certain changes may be made in the above construction withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

What is claimed is:

l. A transmission line crossover comprising a first transmission linehaving a pair of first center conductors disposed between a pair ofground plane conductors, a second transmission line including a secondcenter conductor disposed between said ground plane conductors, firstand second sections in said first line, said first center conductorsbeing substantially in registration at the end points of said first andsecond sections and one of said first conductors being an odd number ofhalf wavelengths longer than the other in each of said first and secondsections, a third section between -said first and second sections, saidfirst conductors being in registration"1n said third section, insulatingmeans disposed between said first conductors in said first, second andthird sections, said second center conductor passing through said firsttransmission line between said fu-st conductors in said third section.

2. The combination defined in claim 1 in which said second centerconductor is disposed in a plane substantially midway between said firstconductors in said third section.

3. A transmission line crossover comprising a first transmission linehaving a pair of first center conductors disposed between a pair ofground plane conductors, a second transmission line including a secondcenter conductor disposed substantially midway between said ground planeconductors, said first line including a pair of conversion sectionsconverting propagation along said line between a strip line mode inwhich said rst center conductors are at substantially the same potentialand a parallel plate mode in which said first center conductors are atsubstantially equal and opposite potentials with respect to each other,said conversion sections being spaced apart along said firsttransmission line to form a parallel plate section between them, saidsecond center conductor passing through said first line between saidfirst conductors in said parallel plate section.

4. The combination defined in claim 3 in which said second centerconductor is disposed in a plane substantially midway between said firstconductors in said parallel plate section.

5. A transmission line unit comprising a pair of ground plane conductorsand a pair of rst center conductors disposed between them, a firstsection in which said center conductors are in registration at the endpoints thereof, one of said first conductors being an odd number of halfwavelengths longer than the other in said first section, a secondsection similar to said first section, a third section of said linedisposed between said first and second sections, said center conductorsbeing in registration in said third section.

6. A transmission line crossover comprising a irst transmission linehaving a pair of ground plane conductors and a pair of first centerconductors disposed between them, `a second transmission line Iincludinga second center conductor disposed between said ground plane conductors,a first section in which said center conductors are of unequal length, asecond `section spaced from and connected to said -rst section through athird section, said center conductors being of unequal length in saidsecond section and of equal length in said third section whereby thetotal length of said irst center Iconduct-ors in `said sections areequal, said second center conductor passing through said rsttransmission line between said conductors in said third section.

References Cited in the file of this patent UNITED STATES PATENTS2,155,652` Gothe et al Apr. 25, 1939 2,401,717 Wolff et al June 4, 194610 `2,865,006 Sabaro Dec. 16, 1958

1. A TRANSMISSION LINE CROSSOVER COMPRISING A FIRST TRANSMISSION LINEHAVING A PAIR OF FIRST CENTER CONDUCTORS DISPOSED BETWEEN A PAIR OFGROUND PLANE CONDUCTORS, A SECOND TRANSMISSION LINE INCLUDING A SECONDCENTER CONDUCTOR DISPOSED BETWEEN SAID GROUND PLANE CONDUCTORS, FIRSTAND SECOND SECTIONS IN SAID FIRST LINE, SAID FIRST CENTER CONDUCTORSBEING SUBSTANTIALLY IN REGISTRATION AT THE END POINTS OF SAID FIRST ANDSECOND SECTIONS AND ONE OF SAID FIRST CONDUCTORS BEING AN ODD NUMBER OFHALF WAVELENGTHS LONGER THAN THE OTHER IN EACH OF SAID FIRST AND SECONDSECTIONS, A THIRD SECTION BETWEEN SAID FIRST AND SECOND SECTIONS, SAIDFIRST CONDUCTORS BEING IN REGISTRATION IN SAID THIRD SECTION, INSULATINGMEANS DISPOSED BETWEEN SAID FIRST CONDUCTORS IN SAID FIRST, SECOND ANDTHIRD SECTIONS, SAID SECOND CENTER CONDUCTOR PASSING THROUGH SAID FIRSTTRANSMISSION LINE BETWEEN SAID FIRST CONDUCTORS IN SAID THIRD SECTION.